The invention relates to a method of demagnetizing a magnetic recording medium, for use in a magnetic recorder, and an apparatus for carrying out the same.
In magnetic recording, magnetic fields leaking from a medium affect a head magnetic field, and a head, thereby inducing a transition shift, and shift in the baseline of a reproducing waveform at the time of recording and reproducing. As a result, there occurs deterioration of not only an error rate in a data region, but also write/read characteristics of information for a servo. Such a phenomenon as described occurs regardless of whether it is the case of a longitudinal magnetic recording system or the case of a perpendicular magnetic recording system, but in the case of the perpendicular magnetic recording system, in particular, leaking fluxes from the medium are prone to act on the head, having a significant effect thereon. In order to avoid occurrence of the phenomenon, it becomes necessary to render remanent magnetization zero by causing the medium to undergo alternating-current demagnetization.
In the past, alternating-current demagnetization for rendering the remanent magnetization zero has been implemented by disposing permanent magnets or electromagnets such that magnetic fields differing in polarity from each other, in the longitudinal direction of a recording layer, that is, an easy direction of magnetization thereof, can be applied to the recording layer, and by moving the permanent magnets away from the medium, or gradually reducing respective current values of the electromagnets while applying the magnetic fields produced by the permanent magnets, or the electromagnets to the medium in rotation, in the case of, for example, the longitudinal recording system. In this process, the magnetic fields applied to the recording layer of the medium are gradually decreased in strength while respective polarities thereof are reversed from each other, and as a result, the magnetization of the recording layer is gradually turned into a zero-magnetization state while the recording layer repeating magnetization reversal, thereby attaining alternating-current demagnetization. With the perpendicular magnetic recording medium as well, there is adopted a magnet configuration such that magnetic fields differing in polarity from each other, in a direction normal to a medium plane, that is, the direction of the easy axis of magnetization of a recording layer, can be similarly applied to the recording layer, and by gradually decreasing respective strengths of the magnetic fields as applied, alternating-current demagnetization can be implemented. Such a system for causing a medium to undergo alternating-current demagnetization has been described in JP-A No. 2001-331904, and JP-A No. 2002-133603, respectively. Further, in JP-A No. 2002-163801, there has been described a structure where magnets are disposed in staggered positions, respectively.